Field of the Invention
This invention relates generally to a fire-resistant, ceiling board supporting grid system, and more particularly, to runner members capable of absorbing compressive elongation without substantial buckling as might occur during abnormal elevated temperatures, thereby preserving the integrity of the ceiling as a fire barrier under such conditions.
One of the critical problems encountered in these supporting grid structures is to maintain integrity under abnormally elevated temperatures, such as during a fire. Under these high temperature conditions, metallic grid members, which generally are fixed at their end points, expand and buckle whereby the supported ceiling panels are displaced and drop through the openings formed by the buckled grid members. As a result, the effectiveness of the suspended ceiling as a fire barrier is destroyed and the support structure is exposed to fire.
Prior art has considered structures for absorbing thermally induced compression in a supporting grid member. Most of the early designs used multiple expansion joints in a main runner. A relatively close placement of expansion joints will perform best in fire. This is because, when subjected to fire, the intersecting cross tees will remain close to their original spacings and thereby continue to support the panels. The early systems, however, were severely weakened at their expansion relief locations and could not be installed efficiently without excessive handling damage. Further, they were weak in cross bending and could not maintain beam alignment when exposed to fire.
To resolve this problem, some of the commercial systems reverted to main beams with only one relief point located near the end of the runner (reference U.S. Pat. No. 3,388,519). This however, reduced the handling problem at the expense of optimum fire performance.
Over the years there have been step-by-step improvements in systems that use multiple relief points in each main runner. However, the above problems, to a lesser degree, remain valid even in the improved systems. U.S. Pat. Nos. 3,778,947, 3,965,631 and 4,606,166 show products that handle marginally well when the web is kept vertical. The crushed bulb of these designs, however, limits handleability. This is especially true when a beam is handled on its side. U.S. Pat. Nos. 4,016,701 and 4,128,978 show products with metal removed from the top of the bead. This is the area of maximum bending stress, and the removal of metal here will significantly reduce load carrying capacity.